In a steam turbine for power generation, a rotor blade installed in a turbine stage of a low-pressure stage is apt to have a blade of an increased length for the sake of improvement of a power generation efficiency and an increase in a power generation capacity. Thus, a high strength, a high toughness, and a high corrosion resistance are required of a rotor blade installed in a turbine stage of a low-pressure stage.
As a material constituting a rotor blade of a low-pressure stage in an existing steam turbine, there is used a ferrous material having a property of a tensile strength of 1300 MPa class as a strength and a Charpy absorbed energy at a room temperature of 40 J class as a toughness. As the ferrous material constituting a rotor blade, one which is more excellent in a strength and a toughness is presently required.
Since a centrifugal stress by a high-speed rotation of a turbine acts on a rotor blade, as for the strength, a specific strength (obtained by dividing a tensile strength by a density) is given a greater importance. Thus, a titanium alloy or the like, which has a small density, is recently used as a material constituting a rotor blade. However, the titanium alloy is expensive and it is desired to substitute an inexpensive ferrous material for the titanium alloy.
As a ferrous material having a high strength, a high toughness, and a high corrosion resistance, there is cited a precipitation hardening type martensitic stainless steel. Study is being done for improving a strength, a toughness, a corrosion resistance and so on of this stainless steel.
In a conventional precipitation hardening type martensitic stainless steel, in general, a toughness is reduced when a tensile strength is improved. Therefore, various elements are added in order to improve the strength and the toughness in a balanced manner. However, a martensitic transformation start temperature is lowered when an amount of the added elements is large, making a retained austenite generated easily at a time of quenching. If an added amount of Cr is increased for the sake of improvement of a corrosion resistance, an δ ferrite is apt to be generated.
As described above, a complicated constraint condition exists for maintaining a stability of a martensite structure in a heat treatment process. A precipitation hardening type martensitic stainless steel having a predetermined strength and toughness is required under such a constraint.
In the conventional precipitation hardening type martensitic stainless steel, a sub-zero treatment is sometimes required in order to complete martensitic transformation in a heat treatment process, for example. There is a problem that a manufacturing cost increases by such a treatment. Further, in the conventional precipitation hardening type martensitic stainless steel, a sufficient strength or toughness suitable as a material for a rotor blade of a low-pressure stage in a steam turbine, for example, is not obtained.